Sealing arrangement for fuel cells

ABSTRACT

A sealing arrangement for fuel cells, including a at least one composite ( 40 ) formed of two cell separator plates ( 1, 4 ; BPP) with a deformable membrane electrode assembly ( 18 ; MEA) placed therebetween, the deformable membrane electrode assembly being composed of two porous, gas-permeable plates or layers ( 2, 3 ; GDL) and an ion-exchange membrane ( 5 ; PEM) placed therebetween, the lateral surfaces ( 7, 8, 9 ) of the membrane electrode assembly being set back with respect to the lateral surfaces ( 6, 10 ) of the cell separator plates to leave a sealing gap ( 19 ), an elastic sealing element ( 17 ) which encloses the composite in the manner of a peripheral sealing band ( 28 ), the sealing element ( 17 ) having a peripheral sealing strip ( 20 ) which extends into the sealing gap ( 19 ) to seal the sealing gap ( 19 ) in a gas-tight manner by compression between the cell separator plates ( 1, 4 ; BPP).

Priority to German Patent Application No. 101 60 905.1-45, filed Dec.12, 2002 and hereby incorporated by reference herein, is claimed.

BACKGROUND INFORMATION

The present invention relates to a sealing arrangement for fuel cellsincluding at least one composite formed of two cell separator plateswith a deformable membrane electrode assembly placed therebetween. Thedeformable membrane electrode assembly is composed of two porous,gas-permeable plates or layers and an ion-exchange membrane placedtherebetween, the lateral surfaces of the membrane electrode assemblybeing set back with respect to the lateral surfaces of the cellseparator plates to leave a sealing gap. The present invention relatesas well to an elastic sealing element enclosing the composite in themanner of a peripheral sealing strip.

Fuel cells are electrochemical energy converters and are well-known.They produce electric energy by oxidizing a fuel. In the simplest case,they are composed of planar, electrically conductive electrodes whichare gas-permeable and separated from each other by an ion-conductingmembrane. The reaction media are supplied via distribution plates havingintegrated gas- or liquid-conveying channels. These distribution systemshave to be sealed both from each other and from the outside. To producean electric voltage or an electric current of a technically usablemagnitude, usually a plurality of large-surface, thin plates or layersare arranged above each other in the form of a stack and the individualcells are interconnected in series or parallel. The electric energyproduced by the converter is tapped at electrically conductiveelectrodes of the stack.

In the simplest case, such an electrochemical fuel cell is composed oftwo electrodes, designed and referred to in literature as a planar “gasdiffusion layer”, hereinafter abbreviated as GDL, between which islocated an ion-conducting layer, each electrode having an adjacent gasspace in which in each case one reaction medium is supplied viadistribution channels. Seals between the individual cell elementsprevent the reaction medium from escaping.

In certain fuel cells, the ion-conducting membrane is a polymer. Thepresent invention relates to the sealing of such a polymer electrolytemembrane fuel cell, hereinafter referred to in short as “PEM cells”.This type of chemical fuel cells is increasingly gaining importance as afuture energy source for the propulsion of motor vehicles. Therequirements for this application include as favorable a mass/powerratio as possible and a sealing of the distribution systems whichremains reliable over several years.

In polymer electrolyte membrane fuel cells, the two porous,gas-permeable electrodes and the very thin proton-conducting polymerelectrolyte membrane placed therebetween are usually combined into aso-called “membrane electrode assembly”, hereinafter abbreviated as MEA.When arranged in the stack, these assemblies are separated by so-called“cell separator plates”. The latter are provided with theabove-mentioned distribution structures for the reaction gases in thesurface. The stack is terminated with end plates on each of the endfaces and held together by tie bolts, pressing the layers together.Often, nonmetals, such as graphite, but also metals, such as high-gradesteel or titanium, are used for the electron-conducting cell separatorplates. A suitable electrode material for the anode or cathode isplastically deformable and electrically conductive material such asgraphite films or non-woven fabric materials. The electrode surfacecontacting the polymer electrolyte membrane is coated with a catalyst,for example, a platinum material. Cell separator plates within the stackare in electrical contact with the anode of a cell of the stack via oneof their surfaces while their opposite surface is in contact with thecathode of another, adjacent cell. According to this function, thesecell separator plates within the stack are also referred to as so-called“bipolar plates”, hereinafter referred to in short as “BPP”. Apart fromtheir function of conducting the electric current in the stack, theyalso have the function of separating the reaction gases.

For a PEM fuel cell, usually, hydrogen is used as the reaction gas andoxygen or air are typically used as the oxidizing agent. Hydrogen issupplied to the anode chamber formed by the distribution structure onthe anode while the oxygen or air is supplied to the cathode chamber.Via the gas-permeable electrodes, the reactants reach theproton-conductive ion-exchange membrane through the catalyst layer.Cations forming at the catalyst layer of the anode migrate through theion-exchange membrane and react with the oxidizing agent supplied at thecathode side to produce, on one hand, water as a reaction product and,one the other hand, electric and thermal energy. The electric energy canbe supplied to a load via a an external electric circuit while thethermal energy in the stack has to be dissipated through suitablecooling channels between the cell separator plates.

High demands are placed on the seals between the individual cellelements. PEM fuel cells which are intended to supply energy to a motorvehicle are exposed to rough environmental conditions. The seal has towithstand heavy vibrations, humidity fluctuations and variations intemperature. Leaks can occur due to different material expansion.

To seal the gas spaces and the fluid collection channels, German PatentApplication No. 197 13 250 proposes a gas- and liquid-tight adhesivecomposite of the membrane electrode assembly with the adjacent cellseparator plates in the manner of a peripheral seal. The adhesivecomposite material is achieved by an adhesive agent which interconnectsthe cell elements in a marginal region; forming a gas-tight seal. Thelateral surfaces of the membrane electrode assembly are set back withrespect to the lateral surfaces of the cell, separator plates, thusforming a sealing gap which is filled by the adhesive composite materialand protects the polymer electrolyte membrane from desiccation. Severalsuch modules can be connected by coating the end faces of the stack withadhesive composite material. The handling of the adhesive agent, whichneeds to be accurately applied in the marginal region, is a disadvantageduring production. Another disadvantage is the undetachable connectionin a stack of fuel cells as a result of which the whole stack must bediscarded when one cell is defective.

SUMMARY OF THE INVENTION

It is an object of the present invention is to provide a sealingarrangement which allows a composite of plates of a fuel cell or of aplurality of fuel cells to be reliably sealed in a gas-tight manner andpermits easy replacement of a defective composite in a stack of fuelcells.

A further alternate or additional object of the present invention is toprovide a manufacturing method which allows mass production at lowproduction cost. Another alternate or additional aim of the presentinvention is to devise an electrochemical energy converter which issuitable for mobile application and allows easy maintenance and repair.

The present invention provides an elastic sealing element which enclosesthe composite in the manner of a peripheral sealing band and features asealing strip which is compressed in a sealing gap between cellseparator plates, thus sealing the sealing gap in a gas-tight manner. Inthis context, the present invention is based on an arrangement of theplates or layers in which the peripheral lateral surfaces of themembrane electrode assembly are set back with respect to the peripherallateral surfaces of the cell separator plates. In this manner, a sealinggap is left at the peripheral lateral surface of the composite. Aprojecting rib of the sealing element, the sealing strip, extends intothis sealing gap. Due to the deformability of the electrodes of themembrane electrode assembly, the sealing gap becomes narrower as soon asa pressing force is exerted on the end faces of the end plates. Exposedmarginal surfaces of the cell separator plates in the sealing gap becomepressing surfaces, resulting in a compression of the elastic sealingstrip located therebetween. The gas-tight elastic sealing materialcompressed between the pressing surfaces of the cell separator platesforms an effective barrier, preventing the reaction gases from escapingin the compression gap. The compression in the sealing gap also causesthe elastic sealing material to spread laterally, thus increasing thepressure on the peripheral lateral surfaces of the membrane electrodeassembly also laterally. In this manner, a reliable sealing effect isachieved even when the individual plates or layers of the compositedeform or expand due to mechanical stress during assembly or because ofvibrations or thermal expansion during operation. Especially theion-exchange membrane is sensitive to expansion. Since, according to theinventive embodiment, the elastic sealing material contacts only theperipheral lateral surface of the membrane but not the upper or lowersurface thereof, damage to the PEM due to deformation or expansion ofthe plates is nearly ruled out. The enclosing sealing band also preventsthe polymer electrolyte membrane from desiccation. Due to the mechanicalcompression of the peripheral sealing strip, the sealing gap isgas-tight even if the elastic sealing material does not or only verypoorly adhere to the lateral surfaces, which is the case, for example,if the cell separator plates are composed of graphite. Each cell in astack can be provided with pressing surfaces of different size,depending on its position in the middle or end regions of the stack. Inthis manner, the distribution of the sealing pressing force, which isinhomogeneous within the stack, is equalized along the length of thestack.

In the case of a plate composite which is located in the end region ofthe stack and to which increased pressure is applied, unacceptably highcompression of the sealing material in the sealing gap can be preventedby appropriately sized pressing surfaces. Through proper sizing of therespective pressing surfaces, it can be achieved that the sealingfunction is approximately equal in the middle and end regions of thestack although the pressing forces have different magnitudes.Furthermore, the sealing element designed according to the presentinvention makes it possible to combine several cells into composites.Because of this, defective cells can easily be changed in a modularfashion. The production cost of the seal is comparatively low. Due tothe sealing gap extending into the composite at the peripheral lateralsurface, the surface dimensions of the polymer electrolyte membrane andthus the material cost of the fuel cell are reduced. The sealingarrangement according to the present invention increases the totalweight of the electrochemical energy converter only very slightly, whichis advantageous for a mobile application. No depressions are required inthe cell separator plates for the sealing element, which is convenientfor production.

With regard to a simple and inexpensive manufacture, it is of decisiveimportance that the sealing band and the sealing strip be integrallyformed as an injection-molded part of a uniform material made of apolymer. Through injection molding, the elastic sealing materialpenetrates into the smallest areas of the sealing gap, filling itcompletely.

With regard to production costs, it is an advantage that the sealingarrangement is manufactured and installed in one operation. Due to thesealing material which adheres firmly to the lateral surfaces and to thepressing surfaces, the individual layers are not only sealed but alsoheld together.

Advantageously, the sealing element is designed such that extends overan outer edge of an end face of an outer first cell separator plate andover an outer edge of an end face of an outer second cell separatorplate in order to hold together the composite or composites in aclamp-like manner. In this manner, modules are formed. This isparticularly advantageous with regard to maintenance and repair in afuel cell stack, because this allows defective modules to be changed ina simple manner.

In this context, it is advantageous for the sealing element to bedesigned to have a peripheral sealing profile in the region of a firstclamp edge and to be a flat surface in the region of a second clampedge. In this manner, a coolant which circulates between modules caneasily be sealed by the sealing profile.

It is advantageous if the polymer is an elastomer. Elastomers arewidespread in general sealing technology. The materials EPDM (ethylenepropylene diene rubber), FPM (fluorocarbon polymer), TPE (thermoplasticelastomer) are particularly easy to process using injection molding. Itis also conceivable to use silicone or other plastics such as epoxyresin.

A particularly reliable sealing effect can be achieved if the porous,gas-permeable plates of the GDL are each impregnated and/or coated witha second polymer on one or two sides in an end region at the edge of thesurfaces, and the lateral surfaces of the ion-exchange membrane are setback with respect to the lateral surfaces of the porous plates, thusleaving a second sealing gap into which extends a second sealing stripto seal the second sealing gap in a gas-tight manner by compressionbetween the cell separator plates. The compression of the first sealinggap is located before the compression of the second sealing gap. By thismeasure, the reaction gases are reliably sealed off between theGDL-layers of an MEA. Overall therefore, the sealing effect is improved.Here too, the polymer electrolyte membrane makes contact with theelastic sealing material only at its peripheral lateral surface. Incomparison with the known prior art, the sealing surface of the polymerelectrolyte membrane is thus further reduced and material cost isreduced.

For a particularly good sealing effect, it is also advantageous if theporous plates are completely soaked by a second polymer in an endregion. In this manner, the sealing of the reaction gases does notexclusively fall to the sealing gap but is, at least partially, alreadyaccomplished in the porous plate. Suitable materials for the secondpolymer include those made of silicone or FPM (fluorocarbon polymer),epoxy resin or PTFE (polytetrafluoroethylene).

It is particularly advantageous for the first polymer and the secondpolymer to be the same material. In this manner, a chemical combinationoccurs between the material of the sealing element and the secondpolymer with which the porous plates are soaked. This results in a veryreliable and long-lasting sealing effect which withstands even heavyvibrations during mobile operation.

It is convenient for the sealing gap to have a width of about 50 μm to 4mm and for the elastic sealing element to be formed of a material havinga Shore A hardness of 20 to 100.

The sealing arrangement according to the present invention isparticularly suitable for an electrochemical energy converter whichcontains one fuel cell or a plurality of fuel cells arranged as a stack.In the rare case that the energy converter is constituted by one fuelcell, the present invention allows the sealing arrangement not only toseal the plates of the fuel cell in a gas-tight manner but also to holdthem together. In the by far more important case that the energyconverter includes a plurality of fuel cells which are arranged aboveeach other and interconnected in series or parallel, the presentinvention makes it possible to combine several cells into modules, whichfacilitates maintenance and repair.

For mass production at low production cost, the present inventionproposes a method in which:

-   -   a) the marginal regions of two porous plates are coated or        partially impregnated or soaked with a first polymeric sealing        material,    -   b) an ion-exchange membrane is placed between the two porous        plates to form a membrane electrode assembly,    -   c) a unit is formed in that the membrane electrode assembly        formed in b) is placed between two cell separator plates,    -   d) this unit or a plurality of these units is/are inserted in        the form of a stack into the cavity of an injection mold,    -   e) a contact pressure is applied to the end faces of the        inserted unit or units in the cavity, the pressure being so high        that the polymeric sealing material withstands an injection        pressure with a second polymeric material,    -   f) a composite or composites is/are formed in that a melt of a        second polymeric sealing material is injected into the cavity of        the injection mold,    -   g) the melt is solidified,    -   h) the composite or the composites of fuel cells formed in f        is/are removed from the mold,    -   i) the sealing arrangement is further heated or annealed, if        required.

The injection molding of the polymeric sealing material is of decisiveimportance for an economical production of fuel cells. The vulcanizationmold can be removed conventionally and therefore has a simple design.

The sealing arrangement according to the present invention ismanufactured and installed in one production phase. A module formed ofcomposites of several cells can be produced in one injection moldingoperation together with the coolant seal.

In an embodiment of the manufacturing method, the process time isshortened because it is not the porous plates but a membrane electrodeassembly that is coated or partially impregnated or soaked with thefirst polymeric sealing material in a marginal region.

The coating with polymeric sealing material is preferably carried out byscreen printing, particularly preferably by rotary screen printing. Thepolymeric sealing material can be applied in a very particularly simplemanner using stamp printing. The soaking can be carried out in a simplemanner by dipping or injection molding.

Suitable materials for the coating or for the composite include FPM(fluorocarbon polymer), EPDM (ethylene propylene diene rubber),silicone, PTFE (polytetrafluoroethylene), epoxy resin or TPE(thermoplastic elastomer).

A very reliable and very durable seal can be produced if the materialsfor the coating and the composite enter into a chemical combination.This is the case if the same material is used.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to further illustrate the present invention, reference is madeto the drawings, the Figures of which schematically show differentembodiments according to the present invention. The present inventionthus will be explained in greater detail with reference to theseschematic drawings, in which:

FIG. 1 shows a section through the marginal zone of a fuel cell,including an exemplary embodiment of the sealing arrangement accordingto the present invention, and

FIG. 2 shows the marginal zone of a plurality of fuel cells arranged ina stack, including a second exemplary embodiment of the sealingarrangement according to the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a marginal zone of a fuel cell having a sealing arrangementaccording to a first exemplary embodiment of the present invention. Thecomposite 40 formed of plates is enclosed by peripheral sealing element17 in the manner of a peripheral sealing band 28. The composite ofplates is constituted by two external cell separator plates 1, 4 with amembrane electrode assembly 18 placed therebetween. Membrane electrodeassembly 18, in turn, is composed of three further plates, namely, afirst porous plate 2, an ion-exchange membrane 5 and a second porousplate 3. GDL-plates 2, 3 are permeable to the reaction gases supplied indistribution structure 13, 14. Distribution structure 13, 14 isschematically shown in FIG. 1 as recesses in the surfaces of cellseparator plates 1 and 4 facing membrane electrode assembly 18.Peripheral lateral surface 8 of the ion-exchange membrane is set backwith respect to peripheral lateral surfaces 7, 9 of porous plates 2, 3.All three peripheral lateral surfaces 7, 8, 9 are set back with respectto peripheral lateral surfaces 6, 10 of cell separator plates 1, 4. Inthis manner, a sealing gap 19 is formed which devolves in a centralregion into a second sealing gap 21. According to the present invention,sealing element 17 is designed in such a manner that a peripheralsealing strip 20 extends into the sealing gap. In normal use of the fuelcell, outer cell separator plates 1, 4 are pressed together by endplates and tie bolts, which are not shown in FIG. 1. As a consequence,sealing strip 20 is compressed in sealing gap 19 by pressing surfaces29, 30. The material of the elastic sealing element itself isgas-impermeable and the contact pressure in the sealing gap prevents thereaction gases from escaping from distribution channels 13 or 14 or fromporous plates 2, 3 into surrounding external space 37. Second sealinggap 21, which is formed by the offset arrangement of the peripherallateral surfaces of membrane electrode assembly 18, is also filled withthe elastic material of sealing element 17. This second sealing strip 31also undergoes a plastic deformation because the contact pressure of theouter cell separator plates is transmitted to porous plates 2, 3, theseplates transmitting the contact pressure to second sealing gap 21. Inconjunction with impregnated end regions 15 and 16 of porous plates 2and 3, respectively, the contact pressure in second sealing gap 21prevents passage of the reaction gases between the anode and thecathode. The deformability of porous plates 2, 3 essentially determinesthe transmission of the contact pressures into second sealing gap 21. InFIG. 1, the impregnation is indicated by the hatching of areas 15, 16.However, gas diffusion layers 2, 3 can also be soaked or coated with apolymeric material at the respective surface facing the cell separatorplate. The double-side coating or soaking of end regions 35, 36 improvesthe sealing performance in the marginal zone. The coating or soaking,which also faces the cell separator plates 1, 4, is indicated in FIG. 1by hatched areas 15′ and 16′, respectively.

A very durable and gas-tight composite of the plates of the fuel cell isobtained especially when the elastic sealing material of element 17adhesively bonds to peripheral lateral surfaces 6, 10 and to pressingsurfaces 29, 30 and penetrates in the end regions 35, 36 of porousplates 2, 3. Due to the sealing arrangement, the reactants are not onlysealed, but the complex cell structure is held together at the sametime. The weight and cost of the energy converter are reduced becauseadditional mechanical clamps are omitted. The sealing element, which isdesigned in the form of a clamp, can easily be seen in FIG. 1. Elasticsealing element 17 embraces marginal surfaces 11 and 26 of outer cellseparator plates 1 and 4. Clamp edge 23 differs from clamp edge 24 by anintegrally formed sealing profile 25, whereby, in the case of stackedfuel cells, coolant that is passed over end face 12 of cell separatorplate 1 can be sealed in a simple manner. Lower clamp edge 24 has nosealing profile, but is designed as a flat sealing surface 27. Thesealing profile 25 of a unit arranged below lies sealingly against thisflat sealing surface, which is shown in FIG. 2.

FIG. 2 shows a second preferred embodiment of the present invention, inwhich a plurality of fuel cells are arranged to form a stack. In FIG. 2,the modular design of the energy converter can be appreciated well.Elastic element 17 not only holds together composite 40 of plates ofindividual fuel cells, but modular composites 40′ are formed which areheld together by sealing element 17. As already pointed out in thedescription of FIG. 1, clamp edge 23 has a different design compared toclamp edge 24. Coolant carried in coolant channel 34 is sealed fromexternal space 37 by the contact of clamp edges 24, 23. Theconfiguration and the relative arrangement of the peripheral lateralsurfaces of cell separator plates 1, 4, of porous plates 2, 3 and ofion-exchange membrane 8 corresponds to FIG. 1. However, for improvedclarity, not all the reference numerals are used in FIG. 2. Marginalregions 35 and 36 of porous plates 2 and 3, respectively, are soakedwith a polymeric sealing material, which is also indicated by hatchedportions in FIG. 2. The impregnation prevents the reaction gas carriedin the pores of plates 2 and 3, respectively, from escaping laterally.In this manner, not only the elastic sealing material in the sealing gapbut also end regions 35, 36, which are soaked with polymer, arecompressed between the respective pressing surfaces 29 and 30. Theeffectiveness of the sealing arrangement thus is further improved.

For reasons of clarity, catalyst layers, which are arranged at thesurfaces of gas diffusion layers 2 and 3 facing the polymer electrolytemembrane, are neither drawn in FIG. 1 nor in FIG. 2. End plates and tiebolts, which hold together the stack or the fuel cell, are not shown inthe Figures either.

The manufacture of a sealing element 17, which compresses composites 40of fuel cells as shown in FIG. 2, can advantageously be accomplished byinjection molding. The present invention allows elastic sealing element17 to be manufactured and installed in one operation. In this context,membrane electrode assembly 18 is sealed in a gas-tight manner in thesealing gap while at the same time forming a clamp edge 23 or 24 whichholds together module 40′ and prevents the coolant from escaping intoexternal space 37 using a sealing profile 25. Thus, a simple andinexpensive manufacturing method is available for mass production.

1-14. (canceled)
 15. A method for manufacturing a sealing arrangementfor a fuel cell or for a stack of fuel cells, comprising the steps of:a) coating or partially impregnating or soaking a marginal region of twoporous, gas-permeable plates with a first polymeric sealing material, b)forming a membrane electrode assembly by joining two porous plates withan ion-exchange membrane place therebetween, c) forming at least oneunit by joining two cell separator plates with the membrane electrodeassembly formed in step b) being placed therebetwee, d) inserting theunit or a stack formed of a plurality of the units into a cavity of aninjection mold, e) pressing the unit or the units in the cavity with acontact pressure until the first polymeric sealing material is capableof withstanding an injection pressure with a second polymeric sealingmaterial, f) forming a composite or composites by injecting a melt ofthe second polymeric sealing material into the cavity of the injectionmold, g) solidifying the melt, and h) removing the composite or thecomposites of fuel cells formed in f) from the mold.
 16. The method asrecited in claim 15 further comprising further heating or annealing thesealing arrangement.
 17. A method for manufacturing a sealingarrangement for a fuel cell or for a stack of fuel cells comprising thesteps of: a) coating or partially impregnating or soaking a marginalregion of a deformable membrane electrode assembly with a firstpolymeric sealing material, b) forming at least one unit by joining twocell separator plates with the membrane electrode assembly formed instep a) being placed therebetween, c) inserting the unit of a stackformed of a plurality of the units into a cavity of an injection mold,d) pressing the second unit or units in the cavity with a contactpressure until the first polymeric sealing material is capable ofwithstanding an injection pressure with a second polymeric sealingmaterial, e) forming a composite or composites by injecting a melt ofthe second polymeric sealing material into the cavity of the injectionmold, f) solidifying the melt, and g) removing the composite or thecomposites of fuel cells from the mold.
 18. The method as recited inclaim 17 further comprising further heating or annealing the sealingarrangement.
 19. The method as recited in claim 17 wherein the coatingin step a) is carried out by screen printing.
 20. The method as recitedin 19 wherein the screen printing is rotary screen printing.
 21. Themethod as recited in claim 17 wherein the coating in step a) is carriedout by a high-pressure process or a stamp printing process.
 22. Themethod as recited in claim 17 wherein the soaking in step a) is carriedout by dipping or by injection molding.
 23. The method as recited inclaim 17 wherein the first or second polymeric material is FPM(fluorocarbon polymer), EPDM (ethylene propylene diene rubber),silicone, PTFE (polytetrafluoroethylene), epoxy resin or TPE(thermoplastic elastomer).
 24. The method as recited in claim 17 whereinthe first and second polymeric materials are the same.
 25. The method asrecited in claim 15 wherein the coating in step a) is carried out byscreen printing.
 26. The method as recited in claim 25 wherein thescreen printing is rotary screen printing.
 27. The method as recited inclaim 15 wherein the coating in step a) is carried out by ahigh-pressure process or a stamp printing process.
 28. The method asrecited in claim 15 wherein the soaking in step a) is carried out bydipping or injection molding.
 29. The method as recited in claim 15wherein the first or second polymeric material is FPM (fluorocarbonpolymer), EPDM (ethylene propylene diene rubber), silicone, PTFE(polytetrafluoroethylene), epoxy resin or TPE (thermoplastic elastomer).30. The method as recited in claim 15 wherein the first and secondpolymer materials are the same.